In 1970, Masson began studying to become a psychoanalyst at the Toronto Psychoanalytic Institute, completing a full clinical training course in 1978. His training analyst was Irvine Schiffer, a well-known Toronto analyst and author of books on the unconscious aspects of charisma and time. In 1990 Masson published an autobiographical book in which he accused Schiffer of cursing and being late for sessions.[4] Schiffer denied it and debated Masson on the Canadian television program The Fifth Estate.[5]

During this time, Masson befriended the psychoanalyst Kurt Eissler and became acquainted with Sigmund Freud's daughter Anna Freud. Eissler designated Masson to succeed him as Director of the Sigmund Freud Archives after his and Anna Freud's death. Masson learned German and studied the history of psychoanalysis. In 1980 Masson was appointed Projects Director of the Freud Archives, with full access to Freud's correspondence and other unpublished papers. While perusing this material, Masson concluded that Freud might have rejected the seduction theory in order to advance the cause of psychoanalysis and to maintain his own place within the psychoanalytic inner circle, after a hostile response from the renowned sex-pathologist Richard von Krafft-Ebing and the rest of the Vienna Psychiatric Society in 1896 - "an icy reception from the jackasses," was the way Freud described it later to Fliess [6]

In 1981, Masson's controversial conclusions were discussed in a series of New York Times articles by Ralph Blumenthal, to the dismay of the psychoanalytic establishment. Masson was subsequently dismissed from his position as project director of the Freud Archives and stripped of his membership in psychoanalytic professional societies. Masson was defended by Alice Miller[7] and Muriel Gardiner ("While striving not to take sides," Gardiner said, "I consider him a good and energetic worker and a worthwhile scholar").[8]

Masson later wrote several books critical of psychoanalysis, including The Assault on Truth: Freud's Suppression of the Seduction Theory. In the introduction to The Assault on Truth, Masson challenged his critics to address his arguments: "My pessimistic conclusions may possibly be wrong. The documents may in fact allow a very different reading."[9]Janet Malcolm interviewed Masson at length when writing her long New Yorker article on this controversy, which she later expanded into In the Freud Archives, a book that also dealt with Eissler and Peter Swales. Masson sued The New Yorker for defamation, claiming that Malcolm had misquoted him. The ensuing trial drew considerable attention.[10] The decade-long, US$10 million lawsuit came to a close in 1994 when the court ruled in The New Yorker 's favor.[11]

In 1985, Masson edited and translated Freud's complete correspondence with Wilhelm Fliess after having convinced Anna Freud to make it available in full. He also looked up the original places and documents in La Salpêtrière Hospital in Paris,[12] where Freud had studied with Charcot. Masson writes that the scientific community has been largely silent about his views.[4] Several Freud scholars have disputed the traditional story that Freud's seduction theory patients reported having been sexually abused in early childhood, the basis on which Masson built his case.[13]

Since the early 1990s, Masson has written a number of books on the emotional life of animals, one of which, When Elephants Weep, has been translated into 20 languages. He has explained this radical change in the subject of his writings as follows:

I'd written a whole series of books about psychiatry, and nobody bought them. Nobody liked them. Nobody. Psychiatrists hated them, and they were much too abstruse for the general public. It was very hard to make a living, and I thought, "As long as I'm not making a living, I may as well write about something I really love: animals."[14]

In 2008, Masson became a Director of Voiceless, the animal protection institute. "We are not encouraged, on a daily basis, to pay careful attention to the animals we eat. On the contrary, the meat, dairy, and egg industries all actively encourage us to give thought to our own immediate interest (taste, for example, or cheap food) but not to the real suffering involved... The animals involved suffer agony because of our ignorance. The least we owe them is to lessen that ignorance".[15]